1. Field of the Invention
The present invention relates to portable systems and methods for real-time detection, identification and analysis of trace amounts of chemical materials for such applications as environmental, health and safety monitoring, product quality testing and control, and detection of narcotics, contraband, explosives and chemical and biological agents.
2. Description of the Related Art
Trace detectors are commonly used to detect small amounts of materials, such as, for example, toxic chemicals, product contaminants, counterfeit drugs, narcotics, explosives, chemical and biological materials. Conventional trace detectors rely on collecting and extracting small representative samples of the chemical or biological material of interest (“target analyte”) from air, gas, water, soils, surfaces or other environmental matrices in which the target analytes are usually found. For example, conventional trace explosives detectors rely on the availability of sufficient samples of the explosive compound, chemical precursors or binders, and/or taggants in the form of a vapor or particulate residues on the skin, clothing, baggage and personnel items of people that have come in direct or indirect contact with the explosive. Such reliance is based on the assumption that since many explosive particles have a high sticking coefficient, it is difficult to avoid contamination by the particles when the banned materials are handled, such as during the process of making a bomb or other explosives device. However, the same high sticking coefficient results in extremely low vapor pressures for the particles, which causes detection of the particles to be challenging.
Vapor samples, particle samples or both are acquired for some conventional trace explosives detectors by swiping “suspect” surfaces of luggage, personal items or persons with a swab or the like and placing the swab in the detector. In other conventional systems using portals, pulses of compressed air are directed at the person in the portal to liberate particles off the person's clothing, skin, shoes, etc., and to transport the liberated particles to the detector.
In both types of conventional trace explosives detectors mentioned above as well as other trace detection systems, the sample is introduced into an Ion Mobility Spectrometer (IMS) for analysis. The sample includes analyte molecules (molecules to be analyzed for a potential explosive threat or other target analyte) and background molecules. In the IMS, the analyte molecules and the background molecules are typically ionized using radioactive alpha emitters or beta emitters, and the ions are injected into a drift tube with a constant low electric field (200 volts/centimeter or less) where the molecules are separated on the basis of their respective drift velocities and hence their respective mobilities. The mobilities of the ions are governed by the ion collisions with the drift gas molecules flowing in the opposite direction. The ion-molecule collision cross section of an ion depends on the size, the shape, the charge, and the mass of the ion relative to the mass of the drift gas molecule. The resulting chromatogram is compared to a library of known patterns to identify the substances collected. Since the collision cross section depends on more than one ion characteristic, peak identification is not unique. An IMS system measures a secondary and less specific property of the target molecule—the time it takes for the ionized molecule to drift through a tube filled with a viscous gas under an electric field—and the identity of the molecule is inferred from the intensity versus time spectrum produced by the IMS system. Since different molecules may have similar drift times, an IMS system inherently has less chemical specificity than a mass spectrometer (MS).
IMS-based trace detectors are simple, low cost, and operate at atmospheric pressure; however, IMS-based trace detectors exhibit major shortcomings that make them unsuitable for addressing a variety of emerging threats or other target analytes of interest for various applications. Such limitations include, for example, (1) high false alarm rates due to limited resolution (chemical specificity), which makes IMS vulnerable to interfering molecules, (2) exacerbation of the high false alarm rates near the detection threshold, (3) decreased probability of detection (false negatives) due to higher alarm thresholds, (4) limited and fixed (predetermined) range of detectable explosives threats or other target analytes, and (5) a need to frequently calibrate the detectors by running standards in the presence of various backgrounds.
Recently, alternative technologies have been considered that have requirements that emphasize the reduction of costly false alarm rates, that increase instrument throughput, that implement non-intrusive automated sample collection, and that increase the probability of detection. Mass spectrometers (MS) are strongly considered for use as trace detectors. Because of its powerful analytical capability, a mass spectrometer avoids most of the problems associated with IMS. For example, mass spectrometers are used in a wide range of applications that include environmental monitoring, pharmaceutical drug discovery, and petrochemical processing industries. For trace detection technology, MS-based systems offer some advantages over currently deployed IMS systems. The advantages include lower alarm threshold (by at least a factor of 100) while maintaining low false alarm rates by greater chemical specificity (a factor of 1000 better using medium resolution MS). The uniqueness of mass spectrometry lies in the chemical specificity as the mass spectrometer directly measures a fundamental property of the target molecule—its molecular weight—and thus provides a highly specific means of identifying the molecule. MS-based systems also enable a broader range of target analytes to be concurrently detectable including a broader range of explosives such as peroxide and liquid explosives, chemical warfare agents, and some warfare biological agents. However, progress in the widespread deployment of MS-based detection instruments has been stalled for the past two decades. In particular, mass spectrometers are bulky (e.g., greater than 100 pounds) and costly (e.g., greater than $50,000). Mass spectrometers are also complex instruments that require highly trained personnel to operate the instruments and to interpret the data produced by the instruments. Mass spectrometers require high vacuum to operate, which requires cumbersome vacuum pumps to reduce the atmospheric pressure to a pressure below 1 milliTorr.